As Clarity is a prerequisite to being an essentialist, Book - Essentialism.. the disciplined pursuit of less argues that in order to have the employees at a given company rally behind your mission statement, it needs to be clear.
An easy framework to use for building a clear statement is asking "How will we know when we are done?" or "How will we know when we have succeeded". By answering these questions, your mission statement will have the needed clarity to get your team working towards the same goal.
An unclear goal would be "Improve education in Africa", whereas "Provide 20.000 African schoolchildren with access to stationary" would be measurable and clear, as it directly answers the question asked.